1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus, for which a plurality of exhaust gas purifying catalysts are provided, for purifying the exhaust gases from internal combustion engine, and relates in particular to an exhaust gas purifying apparatus that can reduce the density of the unburnt elements contained in the exhaust to several tens of ppm or lower.
2. Description of the Related Art
It is well known that unwanted elements in exhaust are reduced by providing, for an exhaust system, a plurality of three-way catalysts that perform the oxidation-reduction of elements (HC, CO and NOx) discharged by an internal internal combustion engine, and that when oxygen sensors are provided upstream and downstream of these three-way catalysts, the output of the upstream oxygen concentration sensor is employed to carry out main feedback control for adjusting the air-fuel ratio, and that the output of the downstream oxygen concentration sensor is employed to carry out sub-feedback control for correcting the main feedback control.
In a three-way catalyst, since the unburnt elements (HC and CO) in the exhaust react with water (H2O) and a water-gas reaction occurs whereby hydrogen (H2) and carbon dioxide (CO2) are generated, the hydrogen density tends to be increased downstream of the three-way catalyst that is positioned nearest the downstream end of the exhaust system. Then, when the hydrogen density is increased around the periphery of the oxygen concentration sensor, the sensor output is shifted to a density level that is lower than the actual oxygen density, and as a result, the precision provided by the sub-feedback control is deteriorated.
To resolve this problem, in Japanese Patent Unexamined Publication No. Hei. 8-319822(JP-A-8-319822), a technique is employed whereby the amount of cerium oxide (CeO2), which has an oxygen storage capacity, that each three-way catalyst holds is changed in accordance with the location of the three-way catalyst. That is, the amount of cerium oxide held by a downstream three-way catalyst is smaller than the amount of cerium oxide that is held by an upstream three-way catalyst, so that the water-gas reaction is suppressed and the increase in the hydrogen density downstream of the three-way catalyst is reduced.
However, when the accuracy of the air-fuel ratio control is improved and the exhaust gas purifying function is enhanced as well by increasing the number of three-way catalysts (when the final densities of the HC and CO elements are reduced to several tens of ppm or less), even if the air-fuel ratio is shifted slightly to rich side(on fuel), the densities of the HC and CO elements (oxidize-possible elements, called unburnt elements, generally) in the exhaust are extremely low, as the density of the oxygen is also low at downstream of the three-way catalyst nearest the end of the exhaust system. As a result, there is an insufficient change in the output of the oxygen concentration sensor, and the conventional sub-feedback control can not be carried out. That is, since a so-called binary oxygen concentration sensor, whose output voltage varies widely for the stoichiometric air-fuel ratio, is so designed that its output voltage changes because unburnt elements in the exhaust are oxidized and deposited on the surfaces of platinum electrodes. Therefore, a satisfactory change in the output voltage can not be obtained when the density of the unburnt elements is extremely reduced, and sub-feedback control can be difficult to carry out. In this case, when the water-gas reaction is suppressed using the method shown in JP-A-8-31982, this problem can be partially resolved, but satisfactory effects can not be obtained.
Further, another well known method is one whereby an oxygen concentration sensor is not placed downstream of the downstream end of a three-way catalyst, but in the midst of a plurality of three-way catalysts (for example, when three catalysts are installed on, upstream side of the three-way catalyst nearest the end of the exhaust system). According to this method, however, since the state of the exhaust finally discharged can not be confirmed, the exhaust characteristic may be degraded due to the deterioration of the catalyst located downstream of the oxygen concentration sensor.
And when a binary oxygen concentration sensor is employed, since it will not be able to detect when oxygen saturation of the catalyst in the oxygen storage capacity occurs nor when an excessive amount of oxygen is present downstream of the catalyst, the amount of NOx in the exhaust may be increased.
To resolve this problem, it is one objective of the present invention to provide an exhaust gas purifying apparatus that can implement a low emission characteristic in accordance with which the final density of the unburnt elements is reduced to several tens of ppm or less, and that can appropriately correct a control of an air-fuel ratio based on the output of a sensor that is located downstream of the catalyst in an exhaust system, so that not only an increase in the unburnt elements but also an increase in the amount of NOx in the exhaust can be suppressed.
To achieve the above objective, according to a first aspect of the invention, an exhaust gas purifying apparatus comprises:
a plurality of catalyst purifying means that are positioned in an exhaust system of an internal combustion engine so as to purifying exhaust gas;
upstream side detection means provided upstream of catalyst means located in the upstream end of the exhaust system, for detecting the density of a specific element in the exhaust;
downstream side detection means provided downstream of catalyst means located in the downstream end of the exhaust system, for detecting the density of a specific element in the exhaust;
target air-fuel ratio setting means, for setting a target air-fuel ratio for a gas mixture that is supplied to the internal internal combustion engine;
feedback control means, for carrying out feedback control using the output of the upstream side detection means to match the air-fuel ratio of the gas mixture with the target air-fuel ratio;
first target air-fuel ratio correction means, only when the output of the downstream side detection means exceeds a predetermined value, which corrects the target air-fuel ratio set by the target air-fuel ratio setting means in accordance with the out put of the downstream side detecting means; and
second target air-fuel ratio correction means whcih corrects the target air-fuel ratio to a smaller value than a stoichiometric air-fuel ratio, at least until the output of the downstream side detection means exceeds the predetermined value.
With this arrangement, feedback control is carried out by using the output of the upstream side detection means to correct the air-fuel ratio of the gas mixture so that it matches the target air-fuel ratio. And when the output of the downstream side detection means exceeds the predetermined value, the target air-fuel ratio is corrected in accordance with the output of the downstream side detection means, and further, at least until the output of the downstream side detection means exceeds the predetermined value, the target air-fuel ratio is corrected to obtain a smaller value than the stoichiometric air-fuel ratio. When the output of the downstream side detection means exceeds the predetermined value, it means that the hydrogen density at downstream of catalyst means disposed on the downstream side is high due to the water-gas reaction of the catalyst means. Therefore, only in this case, the target air-fuel ratio is corrected in accordance with the output of the downstream side detection means. As a result, correction of the target air-fuel ratio can be performed in accordance with a slight increase in the unburnt elements, and the density of the unburnt elements in the final exhaust can always be held low. In addition, at least until the output of the downstream side detection means exceeds the predetermined value, the target air-fuel ratio is set to a smaller value than the stoichiometric air-fuel ratio, so that a phenomenon can be prevented whereby there is an excessive increase in the oxygen contained in the exhaust and the NOx in the exhaust is increased. As a result, a preferable exhaust characteristic can be maintained.
The plurality of catalyst purifying means have a purifying function which reduce the density of the unburnt elements downstream of catalyst means disposed on the downstream side to several tens of ppm or lower. It is preferable that the downstream side detection means be an oxygen concentration sensor having a characteristic which varies the output thereof drastically in the vicinity of the stoichiometric air-fuel ratio.
It is also preferable that the second target air-fuel ratio correction means correct the target air-fuel ratio when the internal internal combustion engine is running under conditions of the oxygen storage capacity of the catalyst means frequently tending to become saturated.